Disk drives commonly use spindles to rotate one or more disks that are connected to the spindle. To read or write information on the surface of a magnetic disk, a read/write head or transducer is positioned at a desired location relative to the disk surface. The head flies microinches from the surface of the disk, supported by an air bearing surface. If the air bearing is unreliable and the head is able to penetrate past the air bearing, the head can unwantedly contact the surface of the disk leading to a crashing of the head, speed modulation of the spindle or read/write signal modulation. All of these possibilities seriously impair proper operation of the disk drive. The air bearing can be penetrated as a result of one or more of a number of circumstances. In connection with the present invention, it has been found that flying height modulation can occur due to distortion of the disk caused by clamping forces applied to the disk. This distortion is characterized by either rippling of the disk near the clamping zone or by a bowing or coning of the disk. Ripples can be caused by clamping a disk, which is not flat, against flat surfaces, or by clamping the disk against entrapped contamination. Optical measurements of the disk indicate that ripples commonly occur because of one or more surface irregularities in the disk surface due to processing of the disk. These ripples can be as large as about 100 microinches peak to peak. This problem is especially present on smaller diameter disks due to the thin cross section of such disks. Similarly, thinner disks are much more sensitive to distortion due to clamping forces. The trend toward lower and lower flying heights also exacerbates this problem as it has been demonstrated that lower flying height is more susceptible to modulation due to ripples. When rippling of the disk, particularly near the inner diameter is large enough (peak-to-peak) or of sufficiently short wavelength, the head can contact the disk at the tip of the air bearing surface of the head.
Bowing of the disk refers to a cone-like angle that is defined by the disk surface as it projects outwardly from its inner diameter at the area along which the disk is clamped, such as to a spindle. Bowing results from clamping forces that are non-concentric or, if opposing clamping forces occur at different diameters, a concentrated moment load is produced at the clamping area or disk inner diameter. This moment can result from contamination, out of flatness or taper of the disk or elements that contact or support the disk. As the disk portions become more remote or farther away from the clamping area, the elastic energy associated with the moment dissipates, i.e. the cone-like angle decreases and the surface of the disk no longer appears to have the cone-like shape. Because of the bowing of the disk, when the read/write head flies adjacent to the inner diameter of the disk, different ends of the head can be closer to the disk surface. This causes the magnetic gap to vary in distance from the disk, resulting in signal amplitudes that can vary over a 2:1 range. Such bowing may be controlled or overcome by use of additional mechanical parts or adjustments of mechanical parts alone, but is not necessarily a cost effective solution. Additionally, when the disk is bowed, the tip and tail of the head are closer to the disk and could contact the disk due to disk runout.
The distortion problem is not found, or is not as acute, in larger size disks because the amount of distortion required to cause a problem is a function of the size of the disk and the ratio of the disk size to the head size. Consequently, with disk drives utilizing larger and thicker disks and having essentially the same size head as found in a disk drive with 2.5 inch disks, any wavelength associated with such a comparable ripple on a larger size disk is significantly greater so that the critical wavelength leading to head/disk interference is not so likely to occur.
In order to eliminate, or at least reduce, the distortion of the spinning disk, the amplitude associated with the rippling must be reduced and the uneven clamping forces must also be reduced. The present invention is directed to solving such problems by eliminating or reducing any difference in the surface topography of the disk and the connecting or clamping-related surfaces, which include, in one embodiment, a spacer element that is positioned between two disks that are clamped to a spindle. This is accomplished using a flowable filler material, preferably having tensile properties, that is received by surface irregularities that might be found on the disk surface in or on the spacer element, the clamping area and the inner diameter of the disk. The filler material becomes rigid or hardens so that, after the material fills the surface irregularities and sets, it does not flow out from such irregularities after the disk or disks are clamped to the spindle but, rather, provides a more uniform interface between the disks and its clamping-related parts.
The use of a solidifiable liquid with a floppy disk is disclosed in U.S. Pat. No. 4,387,114 of Conner et al. issued on June 7, 1983, and entitled "Spindle Aperture Reinforcement for a Floppy Disk." The solidifiable liquid acts as a reinforcement member about the central opening of the floppy disk and is intended to reduce or eliminate damage at this area during the course of normal use of the floppy disk. To form the reinforcement member, the solidifiable liquid fills a reservoir defined at the central opening and the liquid subsequently solidifies.